An iceberg about twice the size of New York City is expected to break off an ice shelf in Antarctica, NASA says.

Researchers are monitoring a giant crack in the center of the Brunt Ice Shelf in Antarctica. The crack had been stable for 35 years but has started accelerating toward another fissure called the Halloween crack.

When the larger crack makes its way completely across, it will create an iceberg of at least 660 square miles in a process called calving.

"We don’t have a clear picture of what drives the shelf’s periods of advance and retreat through calving," Chris Shuman, a glaciologist with NASA and the University of Maryland, Baltimore County, said in a statement. "The likely future loss of the ice on the other side of the Halloween Crack suggests that more instability is possible."

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